For quite some time it has been known that lead-acid batteries are particularly suitable motive power applications involving "deep discharge" duty cycles. The term "deep discharge" refers to the extent to which a battery is discharged during service before being recharged. By way of counter example, a shallow discharge application is one such as starting an automobile engine wherein the extent of discharge for each use is relatively small compared to the total battery capacity. Moreover, the discharge is followed soon after by recharging. Over a large number of repeated cycles very little of the battery capacity is used prior to recharging.
Conversely, deep discharge duty cycles are characterized by drawing a substantial majority of the battery capacity before the battery is recharged. Typical motive power applications that require deep cycle capability include Class 1 electric rider trucks, Class 2 electric narrow isle trucks and Class 3 electric hand trucks. Desirably, batteries installed in these types of vehicles must deliver a number of discharges during a year that may number in the hundreds. The cycle life of batteries used in these applications typically can range from 500-2000 total cycles so that the battery lasts a number of years before it needs to be replaced.
Until recently, only lead-acid batteries of the flooded variety have been utilized for the aforementioned deep discharge applications. Flooded lead-acid batteries are designed to have an excess of electrolyte that floods the cell container, completely saturating the plate group and extending into the head space above the plate group to provide a reservoir. The electrolyte reservoir is necessary because as the battery is charged, water in the electrolyte is electrolyzed into oxygen and hydrogen gases, which escape from the cell and deplete the electrolyte volume. To make up for the loss of electrolyte, water must be periodically re-introduced into the cell, or the reservoir must be made large enough to compensate for the expected loss over the life of the battery.
More recently, valve-regulated lead-acid (VRLA) batteries have been introduced that are suitable for deep discharge applications. VRLA batteries rely upon internal gas recombination to minimize electrolyte loss over the life of the battery, thereby eliminating the need for re-watering. Internal gas recombination is achieved by allowing oxygen generated at the positive electrode to diffuse to the negative electrode, where it recombines to form water and also suppresses the evolution of hydrogen. The diffusion of oxygen is facilitated by providing a matrix that has electrolyte-free pathways. The recombination process is further enhanced by sealing the cell with a mechanical valve to keep the oxygen from escaping so it has greater opportunity for recombination. The valve is designed to regulate the pressure of the cell at a predetermined level, hence the term, "valve-regulated".
There are two commercially available technologies for achieving the enhanced oxygen diffusion. One technology makes use of a gelled electrolyte. In gel technology, the electrolyte is immobilized by introducing a gelling agent such as fumed silica. Gas channels form in the gel matrix in the early stages of the cell's life as water is lost via electrolysis. Once the gas channels are formed, further water loss is minimized by the recombination process. Unlike a fibrous matrix, the gel matrix keeps the electrolyte immobilized and there is little bulk movement.
The other technology for enhancing oxygen diffusion makes use of a fibrous material separator between the electrodes. A widely used material for this purpose is an absorbed glass mat (AGM). The AGM is a non-woven fabric comprised of glass micro-fibers that retain the electrolyte by capillary action, but also provide gas spaces as long as the matrix is not fully saturated with electrolyte. The electrolyte is still free to move within the matrix, but is more confined than in a flooded cell. Another fibrous material gaining acceptance is a non-woven mat constructed from a polymeric component such as polypropylene or polyethylene.
One important difference between the fibrous mat and gel technologies, stemming from the degree of electrolyte mobility, is the effect of cell orientation on cycle life. With fibrous mat technologies, particularly when dealing with cells over about 14 inches tall, it has been discovered that the cycle life in deep discharge applications can be significantly improved by arranging the cells horizontally rather than vertically. With gel technology, there is little difference in deep cycle life when cells are arranged horizontally or vertically. Thus, to achieve maximum cycle life, it is necessary to orient fibrous mat cells horizontally, but it is not necessary to orient cell gel cells horizontally. Presumably, this effect can be explained by stratification of the electrolyte in fibrous mat cells when subjected to deep discharge cycling due to the higher degree of mobility compared with gel technology. The stratification results in reduced discharge capacity and can only be reversed with great difficulty.
The benefits of valve-regulated, lead-acid cell batteries of the fibrous mat variety and plate arrangements for deep discharge applications are discussed in U.S. Pat. No. 4,425,412 to Dittmann et al. and U.S. Pat. No. 5,441,123 to Beckley. Although each of the inventions disclosed in these patents takes advantage of horizontal battery plate orientation, the inventions are not without their shortcomings. Dittmann discloses a "monobloc" battery wherein individual cells are not individually formed and enclosed within separate containers. Rather, they are formed by installing plates in a housing having separate cell compartments, and filling each compartment with acid. Individual cell compartments are defined within the battery case between partitions that are sealed to the battery case walls. A significant disadvantage of this approach is the lack of flexibility to adapt the battery configuration to battery compartments of different sizes. That is, a "monobloc" battery constructed with 12 cells will not fit into a battery compartment sized to accept six cells. Moreover, if an individual cell within the monobloc develops a problem, the entire monobloc may be rendered useless or at least significantly degraded in performance. Another disadvantage of the "monobloc" approach is that, for applications requiring large capacity batteries, battery size may increase substantially. This large, heavy battery may be difficult to handle thus raising safety concerns for personnel.
Some of these problems are addressed in Beckley which provides for the prefabrication of individual cells and the placement of individual cells in a preformed compartment in a steel tray assembly. The cell compartments are defined by cell-receiving members (partitions) attached to the tray. This approach is still somewhat limited in that no means are provided for applying compressive force to the cells, other than the fixed space between the partitions. Recent research has demonstrated the necessity of applying and maintaining modest to high levels of compression in fibrous mat cells to keep the separator in close contact with the plates. Having fixed partitions does not readily provide this capability.
In light of the problems with these prior art approaches, there remains a need for a valve-regulated lead-acid battery housing that takes advantage of horizontal orientation performance and that provides individual battery cells, while providing a convenient means of applying an appropriate amount of compression to each cell.